Unlock by passing the "Make the Ultimate Warrior" bill in the Dark Assembly which requires you to beat the final boss, and start a new cycle.

Their Special Ability allows them to have +100% in all stats when all of your allies on the battlefield are dead.

Majins are considered to be the game's best characters. In some cases this is true, but their Movement Range and Throwing ability is the game's worst. Also, their EXP modifier is the highest of the Humanoid characters, making leveling them up a VERY slow grind without Felonies attached. They can utilize any weapon, and can attack in any way possible, be it Physical or Magical (spells will need to be Pupil-taught). However, they truly shine when they are the last character you can summon, as in, all other allies must be dead -- their stats double when this happens, turning them into a one-man army. Aside from weak Movement and Throwing power, Majins must be well-leveled in Weapon Mastery to display their power. If a Majin is the last character and has no weapon skills, they will still be defeated by strong enemies unless over-leveled.